Study On The Diagenesis And Tectonic Implications Of Mafic-ultramafic Rocks In The Western Sector Of The Northern Qilianshan

The North Qilian orogenic belt is between the Alxa block and the Qaidam plate,and is one of the typical accretionary orogenics in the world.There are multiple ophiolite belts are exposed in this unit.The North Qilian ophiolites are mainly divided into southern and Northern belts.The southern belt consists of Aoyougou,Yushigou,Donggou and Dongcaohe ophiolite belts,and Jiugequan,Dachadaban and biandukou constitute the northern belt.In this paper,three ophiolite belts have been discovered in the western part of the North Qilian orogenic belt,which is named as the Kawa ophiolite,the Shuanglong ophiolite and Xiaolongkong ophiolite.Because of them and the Aoyougou-Erzhihala ophiolite has certain space-time relation,therefore,this paper will study the four ophiolite melange belts by geochronology,geochemistry and Nd isotope.Based on the results of previous studies,this paper discusses the nature,structure and genetic relationship of the four ophiolite belts.It provides an important petrological and chronological basis for the early Paleozoic subduction,plate tectonics and dynamics of the northern Qilian.The Kawa ophiolite,the Shuanglong ophiolite,Xiaolongkong ophiolite and Aoyougou-Erzhihala ophiolite were emplaced in the Mesoproterozoic Chang-cheng System of zhulongguan group of Huashugou group,and fault contact.The Kawa ophiolite,the Shuanglong ophiolite,and Xiaolongkong ophiolite belts have the same characteristic.Ophiolite is mainly composed of rock blocks and matrix.The matrix consists of serpentinite,more strongly deformed into powder shape;the rock deformation is weak,mostly of lens.The ultrabasic rocks containing spinel peridotite and mafic components are basalt,diabase and gabbro,lack of cumulates.the Kawa ophiolite,the Shuanglong ophiolite,Xiaolongkong ophiolite and Aoyougou-Erzhihala ophiolite develops diabase walls,and part of rocks and ultrabasic rocks also have a small amount of silicon structures involved in rock and exotic blocks such as tuff,carbonate rock block.According to the thickness of the components of the rock,the thickness of the mantle peridotite in the four stage ophiolite is about 50m,and the intrusion is mainly composed of gabbro and diabase.There are no basaltic outcrops in the Aoyougou-Erzhihala ophiolite,but the other three sections have the output,and generally have pores and almond like structure,no pillow shaped basalt.The Kawa ophiolite,Shuanglong ophiolite and Xiaolongkong ophiolite are the new ophiolite belts,and zircon age is the first time.According to LA-ICP-MS zircon U-Pb dating,The age was 467±2 Ma?MSWD=0.76,Sm-Nd 460 Ma?,474±2 Ma?MSWD=0.53?and 477.6±3.7Ma?MSWD=1.2?.The Aoyougou-Erzhihala ophiolite belts was 480±6.4Ma and 497±3.2Ma.Peridotite component Si O₂ with an average of 41.33%slips in the ophiolite,with low Al₂O₃,Mg O rich Cr,Ni,similar to the"U"type REE patterns,and enrichment of Nb,Ta.Both the ACM and AFM maps are structural mantle peridotite?MP?,and there is no linear correlation with the basic components,and the overall characteristics are similar to that of the upper subduction zone.Studies suggest that they may be spinel lherzolite partial melting of the lithospheric mantle residue.The composition of the peridotite is similar to that of the ophiolite in the Xiaolongkong ophiolite.The contents of Si O₂ and Al₂O₃ were low?average 39.49%and 0.61%?,Mg O?Mg^#=91?,high Cr and Ni content.The ACM and AFM diagrams are also shown to be structural mantle peridotite?MP?.The chondrite normalized REE distribution pattern curve is flat,Ba,Nb,Ta,Pb and enrichment of trace elements,the spinel lherzolite partial melting of lithospheric mantle after finite residue.The peridotite composition of Aoyougou-Erzhihala ophiolite belt is characterized by high Mg O?Mg#=91?,low Al₂O₃?1.23 wt.%?,Ti O₂?mean value of 0.02 wt.%?and poor Ca O?mean value of 0.09 wt.%?.ACM and AFM diagrams are also used to construct the mantle peridotite?MP?.The normalized distribution pattern curve of rare earth element chondrite is weak right type,the relative enrichment of light rare earth,positive Eu anomaly?1.14?4.15?.Nb and Ta are highly enriched and high Cr,Ni content and slip peridotite,dragons hole peridotite,Shi Erhui is spinel peridotites occur after low degree melting of lithospheric mantle residue.Ophiolite geochemistry and isotope geochemistry research results show that the Kawa,Shuanglong and Xiaolongkong ophiolite belts are SSZ type.The basic components?gabbros,diabases and basalts?are similar to the N-MORB,and the spider diagrams show negative Nb-Ta-Ti anomalies.Th/Ta ratios are more than 3,and Nb/U are close to island arc basalt.They show that both the geochemical characteristics of MORB and IAT,the source region are depleted mantle?influenced by subduction?.In the incompatible element discrimination diagram showed that the ophiolite arc basalt,comprehensive is produced in the subduction related arc basin environment.The ancient Qilian oceanic lithosphere subduction southward caused under the back arc secondary expansion,product of partial melting of the mantle wedge.They are different to Yushigou ophiolite belts?N-MOR?.We identified the basic basalts?OIB?in the Xiaolongkong ophiolite.We also identified the back arc basin basalt?both OIB geochemical characteristic?which was strong effect by the deep mantle in the Shuanglong ophiolite.The characteristic of Xiaolongkong OIB basalts have holes enrichment of LREE,HFSE and REE,and no Eu anomaly.They enriched in high field strength elements of Nb,Ta,Ti,Nb/U ratio?mean 59.02?was higher than that of N-MORB?47±10?,and in compatible elements shown in the figure as a product of distinguishing oceanic intraplate environment the.The K₂O content of alkaline basalts in Shuanglong ophiolite?average 0.78 wt.%?is higher than that of the Ophiolites in oceanic tholeiite?<0.3 wt.%?,and the content of Ti O₂?average 0.49 wt.%?than N-MORB?1.5%?.The chondrite normalized REE distribution pattern is similar to OIB,and no Eu anomaly.Their trace element spidergram curve and the ocean similar arc basalts are enriched in Rb,Ba,Th,and U,Pb,Sr,field strength elements Nb,Ta,Ti,compatible elements has enriched mantle evolution trend of discrimination in the source region that basalts are enriched in Shuanglong,and may be the result of deep mantle magma is strongly involved in the process.Ocean island basalts?alkaline basalt?are not the ophiolite components.Probably,on behalf of the ocean island Kawa-Aoyougou arc basin in seamount or oceanic plateaus,positive terrain,in the back arc basin southward subduction preserved proliferation process,and the SSZ type ophiolite rock components with the residues in the ophiolitic mélange.It reveals the Kawa-Aoyougou arc basin basin expansion from subduction resulting in back arc spreading,with the dynamics and the deep mantle uplift.It is as well as in the back arc basin subduction process also exists in the submarine plateau or sea mountain and ocean island of proliferation.Considering the regional geological background and the temporal and spatial relationship of plate tectonics,The newly discovered Kawa-Aoyougou ophiolite belts are a type of suture zone which is completely different of the Yushigou ophiolite belts.It is the result of the subduction and closure of the formation of the Qilian depression in the North Atlantic Ocean,which is caused by the southward subduction of the paleo ocean.But the Yushigou ophiolite belts are the result of the subduction of the paleo ocean in northern Qilian.Kawa-Aoyougou ophiolite belts are the important constraint to the southward subduction of the paleo ocean in North Qilian and the time limit of the island arc magmatism.From the view of the field geology,the strata of the distribution of the ophiolite have the Precambrian small massif.It is suggested that the tectonic framework?back arc basin?is very similar to the Japanese sea.The four ophiolite belts were probably formed in the same mature back arc basin environment.The differences in the chronology suggest that there are at least 40 Ma above the back arc basin,and along with the subduction of the Paleo Qilian Ocean,it was close to the accretion of the micro landmass or island arc.